1. Technical Field
The embodiments herein generally relate to electrical circuits for Sigma Delta data conversion, and, more particularly, to continuous time-sigma delta (CT-SD) converters.
2. Description of the Related Art
Continuous time-sigma delta (CT-SD) converters are exceedingly used in industries, wherever high-resolution conversion is needed at low power consumption. In multi-channel applications these benefits are multiplied and can enable designers to adopt new and beneficial system architectures that were not previously possible. Potential applications for the technology are widespread in all sectors of the electronics industry, particularly where analog signals need to be converted to digital signals in a power-efficient manner, such as in wireless, television (TV) tuners, cell phones, and satellite tuners.
Two parameters that affect the performance of CT-SD converters are stability and dynamic range. Stability refers to the ability of a system to continue converting signals from analog form to digital form for various signal amplitude frequencies and temperature drifts. Instability of a SD converter results in oscillations and a complete loss of the signal. The instability can be caused by large amplitudes of input signals with specific frequency patterns; temperature drifts that result in a variation in the SD loop filter or because of the non-linear nature of the SD loop itself. Dynamic range refers to the ratio of a maximum level of a parameter such as voltage or frequency, to the minimum detectable value of the parameter. CT-SD data converters typically offer clear advantages in terms of power and dynamic range, however conventional CT-SD data converters are conditionally stable as a function of input signal level and frequency contents.
One approach towards addressing the instability is to back off the signal level to the modulator significantly. This represents a margin that reduces the effective dynamic range of the sigma delta. Hence, there is a trade-off, more margin results in loss of dynamic range and a more stable SD. While reducing the margin for the maximum allowable input signal level improves the dynamic range on the expense of stability. In some cases, a stability detector may reset loop filter integrators when instability is detected. Even temperature variations can cause the loop filter response of the CT-SD converter to change leading to such instability even for small input signals. In such cases, the approach of resetting the converter does not address the cause of the instability. The stability detector is typically based on static criteria, such as an assumption of a constant large signal level at the input.
This assumption is inaccurate especially for applications that require signals with a large peak to average, and leads to a significant trade-off on dynamic range, if the circuit is designed to maximize stability. Also, in certain applications (e.g., orthogonal frequency division multiplexing (OFDM), which is used in many wireless applications) there might be a large difference between the signal level peak and the average signal value (e.g., 14 dB), but this large difference may occur only rarely and for small time intervals. The CT-SD converter can handle signals with larger peaks if they statistically occur every once in a while, but because of the margin and due to the circuit being designed for the highest possible signal level, there is a loss of dynamic range.
In certain cases, the circuit may follow a repetitive pattern of instability. In such cases, if the circuit is designed to maximize dynamic range, it does not meet the requirement of stability, and moreover cannot be easily modified to improve stability. Thus, conventional CT-SD converter designs do not typically provide an optimal dynamic tradeoff between stability and dynamic range in different conditions. Accordingly, there remains a need for a CT-SD converter that is reconfigurable adaptively to optimize stability and dynamic range.